An technology relating to an optical transmitter is disclosed in JP 2008-92172 A. JP 2008-92172 A discloses that a modulation scheme such as a CS-RZ modulation scheme, an optical duo-binary modulation scheme, a DPSK modulation scheme, and a DQPSK modulation scheme performs a modulation with an electric signal having an amplitude of 2×Vπ which corresponds to a voltage range including a peak, a bottom and a peak indicated in a drive voltage to light intensity characteristic of an LN modulator. LN is an abbreviated name of lithium niobate. Vπ represents a voltage capable of changing a phase of the LN modulator by π and is called a “half wavelength voltage”.
According to JP 2008-92172 A, in order to control a bias voltage of the LN modulator, a signal with a frequency f0 is superimposed on an amplitude of a drive signal and a variation (or a fluctuation) of an amplitude of an output light signal of the LN modulator is detected.
For example, when the bias voltage is at the optimal value, no f0 component arises in the output light signal of the LN modulator. Meanwhile, when the bias voltage deviates from the optimal value, an f0 component arises in the output light signal.
The direction in which the bias voltage deviates from the optimal value (hereinafter, may be referred to as a “bias deviation”) can be detected based on the phase of the f0 component, which is reversed depending on the direction in which the bias voltage deviates from the optimal value. Therefore, the bias voltage can be controlled to the optimal value by adjusting the bias voltage such that no f0 component arises.
Incidentally, the technology to detect a bias deviation by superimposing a low-frequency signal on a drive signal of a modulator is also disclosed in JP 2013-110620 A and JP 2013-88702 A.
In recent years, a transmission characteristic is improved by introducing digital signal processing technology in optical communication. In an optical transmitter, the technology of digital signal processing performing on an optical transmission waveform attracts attention. For example, it is known that a variance (or dispersion) pre-equalization technology to add an inverse dispersion for a chromatic dispersion generated in an optical fiber to a transmission waveform in advance.
In response to a change of the waveform of a transmission signal, the peak drive amplitude for the average amplitude of a drive signal (hereinafter, may be referred to as an “average drive amplitude”) may be increased. In such a case, the average drive amplitude may be set to an amplitude smaller than 2×Vπ to enable light conversion at the peak drive amplitude. However, when the average drive amplitude is set to an amplitude smaller than 2×Vπ, there is a specific amplitude which makes an appropriate bias control impossible. Thus, there is a case where the bias control may be impossible. The specific amplitude is described in, for example, JP 2013-110620 A and JP 2013-88702 A.